20 research outputs found
Quasi-Newtonian dust cosmologies
Exact dynamical equations for a generic dust matter source field in a
cosmological context are formulated with respect to a non-comoving
Newtonian-like timelike reference congruence and investigated for internal
consistency. On the basis of a lapse function (the relativistic
acceleration scalar potential) which evolves along the reference congruence
according to (), we find that
consistency of the quasi-Newtonian dynamical equations is not attained at the
first derivative level. We then proceed to show that a self-consistent set can
be obtained by linearising the dynamical equations about a (non-comoving) FLRW
background. In this case, on properly accounting for the first-order momentum
density relating to the non-relativistic peculiar motion of the matter,
additional source terms arise in the evolution and constraint equations
describing small-amplitude energy density fluctuations that do not appear in
similar gravitational instability scenarios in the standard literature.Comment: 25 pages, LaTeX 2.09 (10pt), to appear in Classical and Quantum
Gravity, Vol. 15 (1998
Weak lensing generated by vector perturbations and detectability of cosmic strings
We study the observational signature of vector metric perturbations through
the effect of weak gravitational lensing. In the presence of vector
perturbations, the non-vanishing signals for B-mode cosmic shear and curl-mode
deflection angle, which have never appeared in the case of scalar metric
perturbations, naturally arise. Solving the geodesic and geodesic deviation
equations, we drive the full-sky formulas for angular power spectra of weak
lensing signals, and give the explicit expressions for E-/B-mode cosmic shear
and gradient-/curl-mode deflection angle. As a possible source for seeding
vector perturbations, we then consider a cosmic string network, and discuss its
detectability from upcoming weak lensing and CMB measurements. Based on the
formulas and a simple model for cosmic string network, we calculate the angular
power spectra and expected signal-to-noise ratios for the B-mode cosmic shear
and curl-mode deflection angle. We find that the weak lensing signals are
enhanced for a smaller intercommuting probability of the string network, ,
and they are potentially detectable from the upcoming cosmic shear and CMB
lensing observations. For , the minimum detectable tension of
the cosmic string will be down to . With a
theoretically inferred smallest value , we could even detect the
string with .Comment: 39 pages, 5 figures, v2: references added, minor corrections, v3:
matches version published in JCA
Multidimensional Computerized Adaptive Testing for Classifying Examinees With Within-Dimensionality
A classification method is presented for adaptive classification testing with a multidimensional item response theory (IRT) model in which items are intended to measure multiple traits, that is, within-dimensionality. The reference composite is used with the sequential probability ratio test (SPRT) to make decisions and decide whether testing can be stopped before reaching the maximum test length. Item-selection methods are provided that maximize the determinant of the information matrix at the cutoff point or at the projected ability estimate. A simulation study illustrates the efficiency and effectiveness of the classification method. Simulations were run with the new item-selection methods, random item selection, and maximization of the determinant of the information matrix at the ability estimate. The study also showed that the SPRT with multidimensional IRT has the same characteristics as the SPRT with unidimensional IRT and results in more accurate classifications than the latter when used for multidimensional data